Metals, suitably noble metals, are used in many industrial processes for catalytic purposes, in elemental form or as oxides on a support. Porous or non-porous materials may be used as catalyst supports.
When impregnating support materials with aqueous metal salt solutions and then reducing the salt, the metal may be deposited onto the support in the form of an outer shell which completely encases the support. These shell-type catalysts are particularly suitable, such as for selective hydrogenations and for oxidation reactions using oxygen in the gas phase.
One area of application of particular interest is the selective hydrogenation of acetylene, in particular the purification of a hydrogen chloride gas stream in the oxychlorination process circuit for preparing vinyl chloride. Vinyl chloride is prepared from 1,2-dichloroethane, either purely thermally or in the presence of catalysts at elevated temperature, with the elimination of hydrogen chloride. After isolating most of the vinyl chloride, the resulting hydrogen chloride gas generally contains up to 3,000 ppm v/v of acetylene. Before returning this hydrogen chloride to the oxychlorination process, in an intermediate stage as much as possible of the unwanted acetylene has to be removed from the gas mixture. This is carried out by catalytic hydrogenation, wherein the essential requirement is the removal of as much of the acetylene as possible (residual acetylene concentration <30 ppm v/v) by selective hydrogenation of the acetylene to give ethylene which can be used in the oxychlorination process. The process conditions for purification of the hydrogen chloride gas stream mentioned above are described in detail in European patent No. 0052,271 B1 and U.S. Pat. No. 4,388,278. Hydrogenation is generally performed with a clear excess of hydrogen in the temperature range 120° C.–180° C., at a pressure of from 6 to 20 bar absolute.
Fixed bed catalysts are used to purify the hydrogen chloride gas stream by selective hydrogenation of the acetylene contained therein. This is suitably carried out in the presence of palladium-containing catalysts. Catalyst formulations on aluminum oxide have been disclosed for this use but they are restricted with regard to performance potential/selectivity and durability.
A fixed bed catalyst is described in German patent No. 3,037,047 C2, which is prepared by impregnating a silicon carbide support with a solution of a palladium salt, and then drying and reducing the palladium salt with hydrogen. This involves the use of a relatively expensive support material.
European patent No. 576,944 A1 discloses shell-type catalysts prepared by depositing an alloy by PVD (physical vapor deposition) and/or chemical vapor deposition (CVD) on a molded article.
Nonporous molded articles made of glass, quartz glass, ceramic, titanium dioxide, zirconium dioxide, aluminum oxide, aluminum silicates, borates, steatite, magnesium silicate, silicon dioxide, silicates, metal, carbon, e.g. graphite, or mixtures of these materials can be used as supports. The alloy layer deposited on the molded article contains at least one metal which is suitably very easily oxidized, such as silicon, aluminum, zirconium or titanium. The thickness of the layer is suitably from 100 nm to 50 μm.
Finally, European patent No. 755,718 A1 describes a process for preparing loaded nonporous support materials in which nonporous inorganic support materials can be coated with metal and/or metal oxide in an abrasion-resistant manner by applying highly dilute, aqueous solutions of at least one metal compound to the heated molded article in such a way that the water evaporates immediately and then optionally calcining at elevated temperature.
German patent No. 3,200,483 A1 discloses a process for preparing silica-containing molded articles in which a mixture of soluble and insoluble silicas are applied to an inorganic substrate. The resulting molded articles can be used as supports for a variety of catalytic materials.
A catalyst for use in the case of hydrogen chloride gas purification by selective hydrogenation of acetylene has also been described, with silicon dioxide used as the support material (Chem.-Ing.-Tech. 59 (1987) No. 8, pp 645–647). This catalyst is, like the other formulations, limited with regard to its performance (space velocity characteristics).